Blaziken: Stick to physical attacks, or a physical mixer (physical attacks with one special attack). Blaziken has great physical coverage in High Jump Kick and Flare Blitz alone. Add in Stone Edge, and you have yourself one dangerous chicken! You can even throw in Brave Bird if you want (though it really isn't neccesary). Blaziken also gets access to reversal. For non DW variants, Flame Charge can be used to give Blaziken some more speed to destroy stuff. Don't really bother with special attacks (unless its the mixer set) since while Blaziken may have the best special attack of the three (110 compared to Infernape's 104 and Emboar's 100), it has the worst coverage.
Infernape: Can really do anything you want it to, it is just that good. However, Infernape should stick to special attacks or use a special mixer set (special attacks with one physical attack) as both Blaziken and Emboar have better physical options with more attack and stronger moves. However, Infernape does have three physical moves worth using that the other two don't, Fake out (through breeding), Mach Punch (which is priority), and Close Combat (most reliable fighting move). These moves are best used on a mixer set. Infernapes attacking stats are both tied at 104, which gives Infernape the worst attack (compared to Blaziken's 120 or Emboar's 123), but the second best special attack. Infernape is also the only one to get access to both Vacuum Wave and Grass knot, which helps for a special movepool. Getting Grass knot also means that Infernape can use hidden power for a different type instead of grass for good coverage (Ice comes to mind). Infernape is also the fastest of the three with a base 108 speed (compared to Blaziken's 80 and Emboar's 65) which means that it can kill things first that the other two cannot (Salamence and Garchomp come to mind).
Emboar: Can take hits better then the other two with a base 110 HP. Emboar has the best attack (beating out Blaziken by three points), but the worst special attack (100, losing to Infernape by four points). For using the Flame Charge strategy, Emboar is the best at it since it has more bulk and more powerful physical moves to take advantage of. Unfortunately, this also makes Emboar a glass cannon, as while he can destroy stuff very quickly, he also won't last very long, due to his most powerful options having recoil on them (Flare Blitz, Wild Charge, Head Smash). For special moves, while it doesn't get Vacuum wave (due to not existing in the 4th generation), it does get Scald, a water type move which can help against other fire types. As far as fighting type attacks go, Emboar has the least reliable options. Arm thrust is too luck based to get the maximum amount of hits, leaving you with a low base power, Brick Break is just low in power compared to the more powerful options, Hammer Arm (move relearner) takes away speed, and Super power (through breeding) takes away attack and defense.
So to summarize:
Tank: Emboar Speedster, special attacker: Infernape physical attacker, reversaler: Blaziken
When I heard Blaziken was going to get Speed Boost I thought he still wouldn't be a threat, but I was so wrong. SP Blaziken >>>>>> Infernape
With his newly changed Hi Jump Kick (130 Base Power) it screws anything not resistant/immune to fighting with a +2, just watch out for pokes with Protect as it's cause you to miss and due to that you'll get damaged ;( --- "You got to be careful if you don't know where you're going, because you might not get there." - Yogi Berra
Charizard and Typhlosion don't count, since they aren't Fire/Fighting types. But for them, Charizard generally has one of three options, Bellyzard, mixzard (either physical or special), or special sweeper. Typhlosion's bread and butter is using Eruption (often with Choice Specs/Scarf).
I also forget to mention that Infernape also gets U-turn, which comes in handy for scouting and hit and runs.
My point though, is that all three are good, just use the one that fits your playstyle more.